Acoustic infinite sections are used to define the section properties of two-dimensional, three-dimensional, and axisymmetric regions that model an acoustic medium undergoing small pressure changes. You use acoustic infinite sections when you want to improve the accuracy of analyses involving exterior domains. You must create a reference point to indicate the reference node required by acoustic infinite elements. For more information, see “Infinite elements,” Section 22.2.1 of the ABAQUS Analysis User's Manual.
To specify acoustic infinite section properties:
From the main menu bar, select Section
Create.
A Create Section dialog box appears.
Tip:
You can also click Create in the Section Manager or select the create section tool 
in the Property module toolbox.
Enter a section name of your choice. For more information on naming objects, see “Using basic dialog box components,” Section 3.2.1.
Select Other as the section Category and Acoustic infinite as the section Type, and click Continue.
The section editor appears.
Select an acoustic medium material for the acoustic infinite section. If desired, click Create to create a material; see “Creating or editing a material,” Section 12.6.1, for more information.
Enter a value for the section Plane stress/strain thickness. If the section will be used with a two-dimensional region, you must specify the section thickness. ABAQUS/CAE ignores the thickness information if it is not needed for the region type.
To define the number of ninth-order polynomials that will be used to resolve the variation of the acoustic field in the infinite direction, click the arrow to the right of the Order field to decrease the number of polynomials that will be used (applies only to ABAQUS/Explicit analyses). The default value is 10, which is the value always used in ABAQUS/Standard.
Create a reference point; see “Defining the reference point and thickness for acoustic infinite elements,” Section 22.2.1.2.2 of the ABAQUS Analysis User's Manual, for more information. For detailed instructions, see “Creating a reference point,” Section 46.3.
Click OK to save your changes and to close the section editor.